This disclosure is generally directed to processes for making toner compositions and more specifically directed to emulsion/aggregation processes for making toner compositions comprising a resin and a colorant, and other optional additives.
Emulsion aggregation toners are excellent toners to use in forming print and/or xerographic images in that the toners can be made to have uniform sizes and in that the toners are environmentally friendly. U.S. patents describing emulsion aggregation toners include, for example, U.S. Pat. Nos. 5,370,963, 5,418,108, 5,290,654, 5,278,020, 5,308,734, 5,344,738, 5,403,693, 5,364,729, 5,346,797, 5,348,832, 5,405,728, 5,366,841, 5,496,676, 5,527,658, 5,585,215, 5,650,255, 5,650,256, 5,501,935, 5,723,253, 5,744,520, 5,763,133, 5,766,818, 5,747,215, 5,827,633, 5,853,944, 5,804,349, 5,840,462, and 5,869,215, the entire disclosures of which are incorporated herein by reference.
Two main types of emulsion aggregation toners are known. First is an emulsion aggregation process that forms acrylate based, e.g., styrene acrylate, toner particles. See, for example, U.S. Pat. No. 6,120,967, incorporated herein by reference in its entirety, as one example of such a process. Second is an emulsion aggregation process that forms polyester, e.g., sodio sulfonated polyester. See, for example, U.S. Pat. No. 5,916,725, incorporated herein by reference in its entirety, as one example of such a process.
Emulsion aggregation techniques typically involve the formation of an emulsion latex of the resin particles, which particles have a small size of from, for example, about 5 to about 500 nanometers in diameter, by heating the resin, optionally with solvent if needed, in water, or by making a latex in water using an emulsion polymerization. A colorant dispersion, for example of a pigment dispersed in water, optionally also with additional resin, is separately formed. The colorant dispersion is added to the emulsion latex mixture, and an aggregating agent or complexing agent is then added to form aggregated toner particles. The aggregated toner particles are heated to enable coalescence/fusing, thereby achieving aggregated, fused toner particles.
U.S. Pat. No. 5,462,828 describes a toner composition that includes a styrene/n-butyl acrylate copolymer resin having a number average molecular weight of less than about 5,000, a weight average molecular weight of from about 10,000 to about 40,000 and a molecular weight distribution of greater than 6 that provides excellent gloss and high fix properties at a low fusing temperature.
As described above and in the cited patents, emulsion aggregation toner processes typically include five steps. Those steps include (1) emulsion polymerization to prepare primary polymer particles, (2) aggregation/coalescence in which the primary polymer particles and other particles such as colorant are aggregated together into toner sized particles, (3) wet-sieving in which the toner sized particles are classified to remove coarse particles, (4) washing, and (5) drying. During this process, the two most time-consuming steps, and thus the rate-limiting steps in the process, are the aggregation/coalescence and wet sieving steps.
The aggregation/coalescence step is particularly time-consuming, as three sub-steps are performed in the same reactor. First is homogenation. During homogenation, flocculant or coagulant is added and high amounts of shear are provided to start initial stages of particle formation. Second is a first heating step, in which the reactor contents are heated to about 35 to about 50 or about 60° C., and the polymer aggregation proceeds. Third is a second heating step, in which the reactor contents are heated to about 70 to about 100 or about 120° C., and the aggregated particles are coalesced into final toner particles.
Despite the various emulsion/aggregation processes that are known and used in the art, a need continues to exist for new and improved processes. For example, although emulsion/aggregation processes can provide toner particles having a particular desired particle size with a narrow particle size distribution, the need continues to exist for processes that avoid the generation of coarse particles that must be removed in a subsequent sieving operation. A need also continues to exist for more streamlined production processes, that would allow higher throughput in the emulsion/aggregation process.